The basic construction of these types of sanders, polishers and grinders is well known and comprises an essentially circular sanding disk or platen having a central mounting through a freely rotatable bearing eccentrically mounted on the end of a drive spindle.
Rotation of the drive spindle causes the sanding disk to orbit about the drive spindle. When no external forces act on the disk, the inherent friction in the bearing results in the disk tending to rotate about the spindle axis at full spindle rotation speed. On the other hand, when light pressure is applied to the sanding disk, rotation of the disk can be prevented and the disk merely orbits, as, for example, in a conventional orbit sanding machine.
However, when the sanding disk is pressed onto a workpiece surface, the frictional contact between the pad and workpiece results in a movement of the pad in which it rotates at some considerably lesser speed than the spindle rotation rate, and usually in the opposite direction to the spindle rotation. It also, of course, orbits. This has been found to be a very useful sanding movement and since it has the appearance of being somewhat random, this is the reason for the term "random orbit" as applied to this type of machine.
However, a problem with such machines is that, when there are no external forces acting on the sanding disk and it rotates at full spindle speed, the operator has to be extremely careful when applying the disk to a workpiece, otherwise the inertia of the disk will result in a deep gouge being cut in the workpiece before the disk settles into its, far less aggressive, random orbit movement. One way out of this problem is to apply the sanding disk to the work surface before switching on the sander and so that it never has the opportunity to work up to full rotational speed. However, most users have an instinctive reluctance to do this on the premise (which is untrue in this somewhat unique case) that one should never engage a machine with its load before it has reached its operating speed.
Numerous patents relating to this type of sander address this problem. Most solve it by providing a planetary gear type arrangement between the sanding disk and a housing for the drive spindle. The gear on the disk meshes with that on the housing so that orbiting of the disk results in its gear running around the gear in the housing so that the disk rotates, in the reverse direction with respect to that of the drive spindle, with a speed determined by the-geometry of the gears and eccentricity of the bearing. Examples of such patents are U.S. Pat. No. 4,754,575, WO-A-8909114, U.S. Pat. No. 4,759,152, U.S. Pat. No. 4,727,682, WO-A-8804218, WO-A-9009869, EP-A-0230621, EP-A-0254850 and EP-A-0320599.
The last two differ from the remainder in that EP-A-0254850 employs a rubber friction ring on the disk which can be engaged with a rolling surface on the housing so that only friction, rather than meshing gear teeth, provides the contact between the two. In EP-A-0320599 there is optional physical contact between the gear rings but, when these are not meshed, a magnetic coupling between the disk and housing prevents unconstrained rotation of the disk.
However, all these systems are somewhat complicated and costly to provide and, (with the exception of EP-A-0245850 and EP-A-0230599) essentially destroy the random movement of the sanding disk which characterise the nature of these types of sanding machine. Instead these systems all constrain the sanding disk platen to rotate with fixed speed and direction.
In another prior art patent U.S. Pat. No. 5,018,314 a leaf spring is mounted on the rear bottom edge of the housing and is arranged to contact the edge of the platen as it orbits. In so contacting the edge (at least once, and for at least a part of, each orbit), it has the effect of reducing the rotational speed of the platen. In fact, from an armature speed of 12,000 rpm this arrangement is said to reduce the speed to 1500 rpm. This approach also suffers a number of disadvantages.
If the leaf spring only contacts the platen briefly during each spindle rotation, as described in U.S. Pat. No. 5018314, undesirable vibration can set in. Moreover, the platen tends to accelerate while not contacted and decelerate while contacted by the leaf spring and this results in an erratic movement of the platen. Secondly, although 1500 rpm is sufficiently slow to remove the gouging problem referred to above, nevertheless the platen still seems to be rotating fast, and, of course, half the problem is in satisfying the user that the problem is solved and with this arrangement this aspect is not achieved. Thirdly, the leaf spring contacts the flexible elastomeric surface of the sanding platen and, particularly with the intermittent contact made by the leaf spring, wear of the contact surface is inevitable. Fourthly, with an armature speed of the order of 12,000 rpm, the platen moves back and forth 12,000 times a minute, regardless of how fast it actually rotates, and it is doubtful that the leaf spring can move at this rate to maintain contact with the edge of the platen.